The present invention relates to a multi-stage process for reforming hydrocarbon feedstocks boiling in the gasoline range. The process can be used to make hydrogen, high octane steams for gasoline blending, and benzene, toluene, and/or xylene rich streams for petrochemical use. In particular, the present invention relates to a reforming process wherein the reforming catalyst is highly sulfur sensitive.
The reforming process embraces a number of reactions such as dehydrocyclization, hydrodecyclization, isomerization, hydrogenation, dehydrogenation, hydrocracking, cracking, etc. The desired outcome is the conversion of paraffins, naphthenes, and olefins to aromatics and hydrogen. Usually, the reaction is carried out by mixing a hydrotreated hydrocarbon feedstock with recycle hydrogen and passing the mixture over a reforming catalyst at a temperature of 800.degree.-1050.degree. F. and a pressure of 0-600 psig.
There have recently been developed highly active and selective reforming catalysts comprising a noble metal such as platinum on a zeolite support. These catalysts are particularly effective for the conversion of C.sub.6 -C.sub.8 paraffins to aromatics such as benzene, toluene, and xylenes which may be recovered by extraction for subsequent use in the petrochemical industry. Some of these zeolite catalysts, however, while highly selective, are rapidly poisoned by sulfur.
Nonacidic Pt-L zeolites are a prime example of such sulfur sensitive catalysts. Examples of Pt-K-L zeolite catalysts are described in U.S. Pat. No. 4,104,320 (Bernard et al.), U.S. Pat. No. 4,544,539 (Wortel), and U.S. Pat. No. 4,987,109 (Kao et al.). Pt-Ba,K-L zeolite catalysts are described in U.S. Pat. No. 4,517,306 (Buss et al.). It is disclosed in U.S. Pat. No. 4,456,527 that such catalysts are able to achieve satisfactory run lengths only when the sulfur content of the feed is substantially reduced, for example, preferably to less than 100 parts per billion by weight (ppbw), and more preferably to less than 50 ppbw. The lower the sulfur content of the feed the longer will be the run length.
The patent literature provides several methods that have been specifically identified as suitable for regenerating a highly sulfur sensitive zeolite reforming catalyst that has been contaminated by sulfur. For example, see U.S. Pat. No. Re. 34,250 issued to Van Leirsburg et al. See also European patent disclosure 316,727, which involves pretreating deactivated Pt-L-zeolite catalysts at 500.degree. C. with a halogen compound, such as carbon tetrachloride, and nitrogen. Continuous catalyst regeneration using the technology described, for example, in the report "Continuous reformer catalyst regeneration technology improved" by Roger L. Peer, et al., Oil & Gas Journal, May 30, 1988, is also well known. In the process, the catalyst moves continuously through the regeneration process by gravity, while gas streams steadily flow radially across the catalyst bed. The objective is to provide essentially continuous fresh catalyst performance, thereby giving steady yields. Various other methods for regenerating sulfur contaminated catalysts are also known to those skilled in the art.
Removal of sulfur from the hydrocarbon stream prior to contact with the sulfur sensitive catalyst, however, has received the main focus in maximizing reforming results when using such catalysts. For example, there is provided in the patent literature several ways to obtain ultralow sulfur feedstocks.
U.S. Pat. No. 4,456,527 describes a process wherein the naphtha feed is hydrofined and then passed over a supported CuO sulfur sorbent at 300.degree. F. to produce a feed containing less than 50 parts per billion by weight (ppbw) sulfur.
In U.S. Pat. No. 4,925,549, residual sulfur is removed from a hydrotreated feedstock by reacting the feedstock with hydrogen over a less sulfur sensitive reforming catalyst, converting the residual sulfur compounds to hydrogen sulfide, and absorbing the hydrogen sulfide on a solid sulfur sorbent such as zinc oxide. In U.S. Pat. No. 5,059,304, a similar process is described except that the sulfur sorbent comprises a Group IA or IIA metal oxide on a support. In U.S. Pat. No. 5,211,837, a manganese oxide sulfur sorbent is used.
In U.S. Pat. No. 5,106,484, a hydrotreated feedstock is passed over a massive nickel catalyst and then treated over a metal oxide under conditions which result in a substantially purified naphtha. The metal oxide is preferably manganese oxide and the treatment may be carried out in the presence of recycle hydrogen.
While the sulfur removal techniques of the prior art are effective, they add to the complexity of the reforming process. For example, additional sulfur sorber and recycle-gas sulfur convertor/sorber reactors are necessary, along with their associated catalyst and sorbent materials. In addition, the recycle-gas sulfur convertor/sorber reactors which typically operate under mild reforming conditions may catalyze side reactions causing some yield loss.
Accordingly, any process involving a sulfur sensitive catalyst which can reduce the need for complicated sulfur removal steps would be desirable.
It is therefore an object of the present invention, to provide a novel reforming process which involves a sulfur sensitive catalyst and is relatively simple in its approach to sulfur removal and protection of the sulfur sensitive catalyst used.
Another object of the present invention is to provide an efficient and effective reforming process which involves a sulfur sensitive catalyst.
These and other objects of the present invention will become apparent upon a review of the following specification, the drawing and the claims appended hereto.